In long distance optical fiber communication systems, repeaters are often required between a transmitter in the form of a light source and a photodetector receiver. Repeaters compensate for signal power attenuation in an optical fiber and reshape signals. In the past, repeaters have often included an optical to electrical converter in the form of a photodetector, electronic amplifiers to amplify a converted electrical signal, a signal processor, and an electrical to optical converter in the form of a laser or light emitting diode (LED). Currently, electrical repeaters are being replaced by various types of direct optical amplifiers. Advantageously, these direct optical amplifiers provide large gain (&gt;40 dB), high bandwidth (&gt;1 THz), transparency to bit rate, and the ability to simultaneously amplify multiplexed and bi-directional signals. In addition to replacing repeaters, direct optical amplifiers have other applications such as power amplifiers for boosting transmitted power, compensating for losses due to signal splitting, and as optical preamplifiers for improving receiver sensitivity.
Two types of direct optical amplifiers that perform well are rare earth doped fiber amplifiers and traveling wave semiconductor laser amplifiers. In a semiconductor optical amplifier, electrons and holes are injected into a semiconductor optical waveguide by means of an electrical current. Amplification of a signal occurs as it propagates through the waveguide by the process of stimulated emission in which photons are generated by electron-hole recombination. In rare earth doped fiber amplifier a pump laser at a shorter wavelength than the signal wavelength excites the rare earth ions to a metastable level. Amplification of the signal then occurs by the process of stimulated emission in which the excited ions fall back to the ground state, giving up their energy in the form of a photon that is coherent with the stimulating photon. The signal and pump light are coupled into the doped fiber by a wavelength division multiplexor which typically consists of two fibers melted or polished together.
In an optical fiber transmission line having direct optical amplifiers, optical isolators may be used to prevent reflected light and amplified spontaneous emission noise generated by the amplifiers from damaging the lasers and to prevent multiple reflections that can limit the receiver sensitivity. Multiple reflection induced relative intensity noise at the receiver can reduce an optical amplifier gain to less than 20 dB. Even if there are no reflections present, Rayleigh backscattering (RBS) can limit the amplifier gain to less than 20 dB. An optical isolator is a device that transmits light in one direction while strongly attenuating light in the reverse direction.
However, certain applications require that an optical fiber transmission line support bi-directional signal propagation. Optical time domain reflectometry (OTDR). the standard method used for fault location of fiber networks, transmits pulses into the fiber from one end of an optical fiber transmission line and detects faults by monitoring backscattered and backreflected light at the same end. Bi-directional propagation is also required for applications that transmit light in both directions along the same fiber.
Since optical isolators only allow unidirectional signal propagation they do not allow bi-directional signaling.
U.S. Pat. No. 4,899,043 in the name of Moschizuki et al. issued Feb. 6, 1990, entitled Fault Monitoring System For Optical Fiber Communication Systems discloses a fault monitoring system in a bi-directional optical fiber communication system. The system provides a bi-directional amplifier disposed between first and second optical transmission lines. A first optical signal is transmitted from one side of the transmission line, is amplified by a bi-directional amplifier and directed back to the transmission side, and is monitored. A second optical signal is transmitted from another side, is amplified by a bi-directional amplifier, and is transmitted back to the other side. The system further allows transmission of another signal of a different frequency from one side to the other amplified by the bi-directional amplifier.
U.S. Pat. No. 4,933,990 in the name of Mochizuki et al issued Jun. 12, 1990 disclose an optical privacy communication system in optical fiber communications between many points. Each station is provided with a privacy circuit the privacy circuit comprising a first and second optical branch. An isolator is inserted on the first optical branch, for passing only a signal of a direction from the first optical branch to the second optical branch, and an optical filter inserted in the second optical branch, for passing only an optical signal of a frequency assigned to the station, so that an optical privacy communication is carried out between stations.
In U.S. Pat. No. 4,972,513 in the name of Mochizuki et al. issued Nov. 20, 1990, a mulit-point amplification repeating system is disclosed in which an output of a unidirectional amplifier inserted in a unidirectional repeating optical transmission system is branched and coupled to an input of a unidirectional amplifier inserted in another unidirectional repeating optical transmission system, so that bi-directional optical communication can be performed between many points connected to a plurality of first optical transmission lines.
Although Mochizuki's inventions appear to perform their intended functions, they do not provide isolation of two bi-directional signals being amplified and transmitted on the same optical fiber from one end to the other.